Standing wave linear accelerators with controlled beam current are utilized in a wide variety of medical and industrial applications, including, radiography, radiotherapy, medical instrument sterilization, food irradiation, and dangerous substance neutralization. In such applications, available space is often limited and, hence, it is desirable that the accelerators be compact. For example, in a medical radiotherapy application, an accelerator, electron gun, and target are installed in an x-ray head of a movable gantry which may be moved around a patient lying on a table to direct x-ray radiation at an appropriate location of the patient's body. To achieve a sufficiently large area of irradiation with the required dose uniformity, the distance between the target and the patient should be as large as possible. In order to maximize the distance between the target and the patient, it is advantageous for the accelerator to have a short structure length and, hence, a high accelerating gradient to produce a beam of charged particles having an appropriate energy level in such a short structure.
In typical standing wave linear accelerators often used in such applications, the standing wave linear accelerators comprise multiple accelerating sections with each accelerating section having an alternating series of connected accelerating and coupling cavities that form a biperiodic structure. An injector emits charged particles into an accelerating section and the charged particles are accelerated as they travel in a charged particle beam through the accelerating sections by electromagnetic fields present therein. The electromagnetic fields are created by electromagnetic power (i.e., in the form of radio frequency (RF) waves) that is produced by an RF generator (for example, a magnetron) and delivered to the accelerating sections by feeding waveguides which, generally, comprise hollow pipes having a rectangular cross-section.
Unfortunately, reflections of the electromagnetic wave are often produced in the feeding waveguides with the extent of such reflections being dependent, at least in part, upon the coupling coefficients between the feeding waveguides and accelerating sections. To make matters worse, for an accelerator operating at a particular beam current, there is only one value of the coupling coefficient between a feeding waveguide and an accelerating section at which all of the power of the electromagnetic wave present in the feeding waveguide is delivered to the accelerating section without reflections. Because the coupling coefficient between each feeding waveguide and respective accelerating section is constant and cannot be changed in the known accelerators for operation at different beam currents, reflections are generated which may travel back to and damage the accelerator's magnetron and, hence, all of the power delivered by each feeding waveguide (i.e., in the form of an electromagnetic wave) is not maximally utilized for particle acceleration.
To prevent such reflections from traveling back to the RF generator, some accelerator manufacturers have employed ferrite isolators or circulators to isolate the RF generator from the accelerating sections and feeding waveguides. However, ferrite isolators and circulators are expensive and their use results in RF power losses and, hence, decreased accelerator efficiency. As an alternative to ferrite isolators and circulators, the 3 dB waveguide hybrid junction was developed for use between the RF generator and the feeding waveguides. A 3 dB waveguide hybrid junction, generally, includes two parallel waveguides having rectangular cross-sections such that each waveguide, therefore, has two walls which are wider than the other two walls thereof (i.e., the wider walls being referred to sometimes herein as “wide walls”). One of the wide walls of each such waveguide comprises a common wide wall therebetween which is shared by both waveguides. Therefore, the parallel waveguides are oriented adjacent to one another by virtue of the shared, common wide wall. In addition, a 3 dB waveguide hybrid junction typically includes a coupling hole, or window, in the shared, common wide wall. When installed in an accelerator having two accelerating sections, a first end of the first waveguide of the 3 dB waveguide hybrid junction is connected to the magnetron output and a second end of the first waveguide is often connected to still another waveguide that, in turn, connects to one of the accelerating sections of the accelerator. A first end of the second waveguide of the 3 dB waveguide hybrid junction is connected to a waveguide load which receives electromagnetic power and a second end of the second waveguide is often connected to still another waveguide that connects to another of the accelerating sections of the accelerator.
In operation, the 3 dB waveguide hybrid junction receives input electromagnetic power from the RF generator through the first end of the first waveguide. A first portion of the electromagnetic power travels through the first waveguide to its second end and then to an accelerating section via another connected waveguide. A second portion of the electromagnetic power travels through the coupling window in the junction's common wide wall and into the junction's second waveguide and then travels through the second end of the second waveguide and on to a different accelerating section via another connected waveguide. Reflections of electromagnetic waves received through the second end of the junction's first waveguide are directed through the coupling window and into the second waveguide. Reflections of electromagnetic waves received through the second end of the second waveguide and reflections received through the coupling window are directed through the first end of the second waveguide to the waveguide load, thereby protecting the RF generator from potential damage.
While the 3 dB waveguide hybrid junction serves to protect the RF generator, high electrical fields are present along the junction's wide wall and at the edges of the coupling window therein. Thus, by virtue of the coupling window being positioned in the junction's wide wall, the maximal power of the 3 dB waveguide hybrid junction is limited. Also, the turns or bends in the waveguides that often connect the 3 dB waveguide hybrid junction to the accelerating sections of an accelerator results in the accelerator having larger overall dimensions, making the accelerator less desirable for the applications described above.
Therefore, there exists in the industry, a need for a particle accelerator that is compact, that makes maximal use of electromagnetic power to accelerate charged particles at different beam currents, and that does not include a 3 dB waveguide hybrid junction with limited maximal power, that addresses these and other problems or difficulties which exist now or in the future.